Pulmonary arterial hypertension (PAH), or high blood pressure in the lungs, is common in patients with congenital heart disease. Historically these patients suffered significant morbidity and mortality due to a lack of effective therapies. More recently, advanced therapies which target the mechanisms underlying the development and progression of PAH have been introduced into clinical care. Oral, intravenous, subcutaneous, and inhaled therapies are all available for the treatment of PAH. Patients with PAH are first treated with oral agents (including sildenafil and bosentan). However, if these agents fail to achieve the desired effect for the patient, intravenous or inhaled therapies may be initiated. Combination therapy with multiple agents is common in routine clinical care. However, the most efficacious therapeutic regimen has yet to be delineated. The present study seeks to evaluate the efficacy of one specific regimen: iloprost, an inhaled prostacyclin derivative, used in combination with oral therapy (sildenafil and/or bosentan). Iloprost has been approved by the FDA for use in this patient population. Adults with PAH already receiving oral therapy will be invited to participate in this study. Iloprost will be added to their current therapeutic regimen for a period of three months, with pre- and post-treatment assessments. These will include a cardiopulmonary exercise test, BNP (a blood test), six minute walking distance, and a quality of life questionnaire.